1. Field of the Invention
The invention relates to tanning wastewater treatment, and more particularly to a tanning wastewater treatment and reuse apparatus and method therefore based on nanotechnology and biotechnology.
2. Description of Related Art
According to statistics, China's leather industry wastewater discharged into the environment per year up to more than 100 million tons, accounting for China's industrial wastewater emissions by 0.3%. In light industry, it is ranked No. 3 behind the paper and brewing industries in terms of wastewater discharge. The tanning industry not only consumes a large volume of freshwater but also discharges a lot of wastewater to the environment and it has adverse effects to health and the sustainable development of society as well as poses a serious threat. Thus, it is desired to improve tanning wastewater treatment. Advances in tanning wastewater treatment and water reuse and freshwater resources conservation, either from the point of view resources conservation or from the point of view of environmental protection, are very necessary and are very important.
Leather industry wastewater discharges high concentrations of organic pollutants in the presence of suspended matters, large volume of wastewater, and complicated wastewater components. It contains toxic substances such as sulfur and chromium. In accordance with the production process, the leather industry wastewater consists of seven parts: original leather containing high concentrations of chloride and acid wash water soaking, lime and sodium sulfide containing highly alkaline liming, wastewater containing trivalent chromium blue chrome, tanning wastewater containing tannin and gallic acid, dark brown vegetable tanned wastewater containing fats and oils, skim wastewater, wastewater and degreased stain rinse wastewater in each manufacturing step. Among them, degreasing wastewater, wastewater with removed liming, chrome tanning wastewater pollutions are most serious.
Degreasing waste: the production of pig leather production accounted for 80% of the production of pig fat in water. The fat content is up to 10,000 (mg/L) and CODCr is 20,000 (mg/L). Wastewater effluent accounts for 4% of total fat but the fat effluent oxygen load accounted for 30% to 40% of the total load.
Dehydration liming wastewater: removal of sulfide liming wastewater pollution. Wastewater has CODCr of 2,0000 to 4,0000 (mg/L), BOD5 of 4,000 (mg/L), sodium of 1,200 to 1,500 (mg/L), and pH of 12. The total water removal liming is 10% of wastewater and oxygen consumes 40% of the total load.
Chrome tanning wastewater: chrome tanning wastewater polluted trivalent chromium. In chrome tanning process, chromium adhesion rate is 60% to 70%, i.e., 30% to 40% of chromium salts in water. Wastewater in chrome has Cr3+ 3,000 to 4,000 (mg/L), CODCr 1,0000 (mg/L), and BOD5 2,000 mg/L.
The conventional tanning wastewater treatment technology is a process of collecting and mixing wastewater prior to discharging into the sewage treatment system. But because wastewater contains large amounts of sulfide and chromium ions, it is easy to inhibit microbial. It is more reasonable that the liquid is treated alone, a unified comprehensive wastewater treatment of the process line is implemented, the wastewater is defatted, liming is removed from wastewater, chrome tanning wastewater is processed separately with recovery of valuable resources, and then is mixed with other unified treatments of wastewater.
After concentrating the brightest tanning wastewater, called tanning combined wastewater which has tanning wastewater organic content and sulfur compounds, chromium compounds content, and oxygen consumption, and its pollution is very serious. It is mainly in the following areas:
Chromaticity: Leather Wastewater by larger, mainly by vegetable tanning, dyeing, tanning and chrome gray alkaline waste caused.
Alkalinity: In general, leather wastewater is alkaline. Combine wastewater has a pH value between 8 and 12. The alkalinity is mainly from lime, caustic soda and sodium sulfide used in processes.
Sulfide: Sulfides in tanning wastewater mainly come from soda ash waste removal a small part comes from the flooding of sulfide waste and the soft protein decomposition products. It is easy to produce sulfur-containing waste acids H2S gas. Sulfur sludge under anaerobic conditions will release H2S gas.
Chromium ions: Tanning wastewater is mainly in the form of chromium ions Cr3+. Its content is generally of 100 to 3,000 mg/L. Usually, it first goes through neutralization and sedimentation, then filtration, and finally feeds to combined wastewater pool.
Organic Pollutant: Tanning wastewater has a high content of protein and other organic matter, and a certain amount of reduction substances, so that BOD5 and CODCr are high.
In tanning process, a great volume of wastewater is discharged each step. In each step, after discharging wastewater combined wastewater is collected and the combined wastewater has a pH of between 8 to 12, chromaticity, CODCr, SS, BOD5 concentrations are very high. It is toxic and contains harmful substances. It also has high concentrations of salts. Tanning industry's overall wastewater quality (test average) is that pH is between 8 and 12, chromaticity (times) is between 500 and 3,500, CODCr is between 3,000 and 4,000, SS is between 2,000 and 4,000, NH3−N is between 250 and 300, S2− is between 50 and 100, Cr is between 100 and 3,000, and BOD5 is between 1,500 and 2,000. It is noted that units, except pH and chromaticity, are mg/L.
Currently, tanning wastewater treatment methods include coagulation, adsorption, advanced oxidation technology, direct loop back usage, flotation, acid absorption, catalytic oxidation, and biochemical method. Each method has a number of advantages and disadvantages. Since a single process is very difficult to achieve the desired effect, in practice, it is typical that wastewater treatment depends on the actual situation and a combination of several methods. Wong Chun introduced a Guangdong-based tanning plant using activated sludge flocculation+contact oxidation process for tanning wastewater treatment. Since being produced in December 2003, it brought about a stable treatment effect. The influent CODCr is 3,000 to 3,500 mg/L. The effluent CODCr is about 40 mg/L. The effluent targets were found to meet provincial standard (DB44/26-2001). Zhang Jie et al. applied sequencing batch activated sludge process (SBR) in a Henan-based tanning for wastewater treatment. First, using the chemical method to remove large amounts of toxic substances in the wastewater and some organic matter, and then through the SBR to process biodegradable dissolved organic matter. Designed as daily processing capacity of 800 m3, when the influent CODCr is 2,500 mg/L, the effluent CODCr is 100 mg/L, far below the national standard (CODCr<300 mg/L). The running cost of the item is 0.8 yuan/ton. The results show that, with the SBR processing tanning wastewater, it is adapted to improve water quality and increase resistance to shock loading capability, especially for the relative concentration of tanning wastewater discharging and water changing characteristics. Moreover, SBR treatment is supported by the provincial investment, and thus operating cost is lower than normal activated sludge process. Gu Qiuping et al. use contact oxidation processes in a Shenyang-based tanning wastewater treatment facilities to transform not only the treated wastewater in order to meet emission requirements, but also improve the processing power and effect. The recovery of Cr3+ is 80% so that a portion of the treated wastewater can be reused. When influent COD is 3,647 mg/L, after the treatment, the effluent CODCr concentration is 77 mg/L below the Liaoning Province “DB21-60-89” extension and revised standards (CODCr<100 mg/L). Yang Jianjun, Gao Zhong Bo, and Jie Shao of the trial artillery battalion Xinji leather factory use materialized+oxidation ditch process to modify the original jet aeration sewage treatment system. Its capacity, after the transformation of the treated water, is increased to 4,800 m3/day. Influent CODCr is 6,100 mg/L and it is an effective wastewater treatment. Actual operations show that the transformation process of the treatment efficiency is high, water quality meets the national “Integrated Wastewater Discharge Standard”. Taoru Jun introduces it to Zhejiang Industrial Zone by using a tanning coagulation+hydrolytic acidification+CAST technology, right from the preparation to tanning and other wet processing section of the integrated wastewater treatment. Designed for maximum water flow of 6,000 m3/day, sulfur ions in wastewater is by pre-aeration, and in the reaction cell plus FeSO4 and coagulant PAC, thereby precipitating removal of Cr3+ by reaction with NaOH in order to precipitate the removal of the reaction. Biochemical and oxygen, and aerobic treatment using a combination of techniques, and oxygen using contact acidification process, which can improve the biodegradability of wastewater while removing a portion of CODCr and SS. Aerobic processes using CAST modified SBR process, with increased organic removal efficiency, impact load capacity and other characteristics. Sunya Bing et al. in China Patent CN100371268C disclose a tanning wastewater treatment by electrolysis method. The treated wastewater CODCr removal rate is 60% to 80%, ammonia nitrogen removal rate is 50% to 70%, sulfur removal rate is at least 95%, suspended solids removal rate is 70% to 80%, and chromaticity removal rate is at least 85%. Coli killing rate is more than 99%. However, this method has a high anode consumption and a high energy consumption.
In summary, the conventional methods has drawbacks including multi-material consumption, high emissions from sludge, wastewater treatment having not reached the industrial wastewater reuse standards for wastewater discharge, high cost and complex operation, easy to produce secondary pollution, and creating a number of difficult issues to be solved. Thus, there is a need of low material consumption, sludge discharging a small volume of wastewater after treatment to be performed in the water reuse. Also, there is a need of low cost, new wastewater treatment methods with easy operation in order to facilitate the production of lower leather material consumption per unit of product, save fresh water resources, and protect the environment.